Use host toolchain and explicit target

This fixes a problem of installing i686 toolchain on an x86_64 host.
Instead, we use the x86_64 toolchain, but add the i686 target and force
all cargo commands to use that target using the `CARGO_BUILD_TARGET`
environment variable.

Fixes: mmtk#1107

Author: wks

.github/workflows/minimal-tests-core.yml

@@ -42,12 +42,24 @@ jobs:
     name: minimal-tests-core/${{ matrix.target.triple }}/${{ matrix.rust }}
     runs-on: ${{ matrix.target.os }}
 
+    env:
+      # This determines the default target which cargo-build, cargo-test, etc. use.
+      CARGO_BUILD_TARGET: "${{ matrix.target.triple }}"
+
     steps:
       - uses: actions/checkout@v4
       - name: Install Rust
         run: |
-          rustup toolchain install ${{ matrix.rust }}-${{ matrix.target.triple }}
-          rustup override set ${{ matrix.rust }}-${{ matrix.target.triple }}
+          # "rustup toolchain install" should always install the host toolchain,
+          # so we don't specify the triple.
+          rustup toolchain install ${{ matrix.rust }}
+          rustup override set ${{ matrix.rust }}
+
+          # Ensure we install the target support for the target we are testing for.
+          # This is especially important for i686-unknown-linux-gnu
+          # because it's different from the host.
+          rustup target add ${{ matrix.target.triple }}
+
           rustup component add rustfmt clippy
 
       # Show the Rust toolchain we are actually using